The invention generally relates to a process for reduction in the ignition temperature of diesel carbon out of the exhaust gases of diesel engines.
The normal operation of a diesel engine results in the generation of carbon soot particles and various other carbonaceous particulates (some of which are known carcinogenics) commonly referred to collectively as "particulates". In addition, various other harmful substances are formed, including nitric oxides and carbon monoxide.
It is highly desirable to remove the major portion of these substances before allowing the exhaust gases to escape into the atmosphere, and extensive research and development has been performed by parties interested in finding a solution to this problem.
It is well known in the art that a filter can be installed in the exhaust gas stream in order to trap the particulates formed during the operation of the diesel engine. Such filters can be made from a variety of materials but are usually constructed from a ceramic or metal monolithic structure. The most common form of the filter is a porous ceramic monolith having a plurality of parallel ducts which extend through the structure except that each duct has only one opening. Thus, the ducts which are open at one end are closed at the other. In addition, when viewed from each end, it will be observed that each open duct is surrounded by ducts which are sealed. Exhaust gases entering one end of a ceramic monolith enter the open ducts but must permeate the porous walls into adjacent ducts in order to exit the filter. In doing so, a portion of the particulates are trapped within the pores of the duct walls. The pore size of the duct walls will determine the quantity of particulates retained and hence the efficiency of the trap. The main ingredient of the particulates which adhere and are trapped in the filter is carbon. As the mass of particulates trapped on the filter increases the exhaust gases are impeded by the pore blockage causing an increase in pressure. This in turn results in an increased back pressure on the engine and a reduction in engine efficiency. It is therefore desirable that the particulates be removed from the filter before such a condition causes operational problems, or alternatively that the filter is removed and replaced with one which is free of particulates. Economics and the practicalities of operating a filter trap device dictate that the removal of the particulates should be carried out in situ with the preferred method of removal being by combustion. The ignition temperature of carbon particulates is approximately 550.degree. C. When the exhaust gases and filter are heated to temperatures higher than 550.degree. C., the particulate becomes carbon dioxide. These ignition temperatures are only achieved if a truck engine is operated under heavy load conditions such as those which may be encountered when climbing a steep gradient or at higher speeds than the 55 miles per hour permitted by current legislation. On level grades, the exhaust gas temperature is often below the ignition temperature of the particulates or soot and hence a progressive buildup occurs on the filter. As an example, the temperature of the exhaust gas of a diesel engine used in ordinary automobiles when operated at normal speeds is approximately 400.degree. C. The unpredictability of establishing consistently high exhaust gas temperatures to prevent excessive buildup of soot can give rise to the problems previously described.
In order to overcome this problem, it has been suggested that the filter can be periodically heated by electrical means to a temperature sufficiently high to burn off the retained soot. The use of such a device is not without risk, for as noted in U.S. Pat. No. 4,319,896, care must be exercised during burnoff of the soot particulates as the possibility exists that excessively high temperatures may develop which could damage the filter.
Another possible solution to the problem provides for the deployment of a burner in the exhaust system ahead of the filter which is ignited as necessary to raise the exhaust gas temperature to the desired level of carbon burnoff. Such a burner is shown by U.S. Pat. No. 4,622,810. Accordingly, a sophisticated exhaust gas cleaner available today comprises, in addition to the filter, a burner adapted to be regularly and at periodic intervals operated to heat the exhaust gases to a temperature high enough to allow the carbon particulates trapped in the filter to be combusted. Examples of cleaners of the type described above are disclosed, for example, in Japanese Laid-open Patent Publication No. 49-71315, published July 10, 1974; U.S. Pat. Nos. 4,167,852 and 4,335,574; Japanese Patent Publication No. 57-38765, published Aug. 17, 1982; and U.S. Pat. Nos. 4,345,431, 4,327,111 and 4,604,868.
Each of these methods of solving the soot build up problem are also associated with an increase in fuel consumption which detracts from one of the major advantages of the diesel engine.
Other methods which have been devised to prevent an excessive soot buildup on the filter involve the use of catalytic materials which are deposited on the monolithic structure and cause a lowering of the ignition temperature of the carbon. Various individuals have tested combinations of catalytic agents to achieve this end and many have been successful. However, the extent to which the temperature reduction takes place is insufficient to ensure that burn off will occur and a temperature differential still exists between the "normal" exhaust gas temperature and the carbon ignition temperature.
It is known in the art that the propensity for a combustible material to ignite increases as the partial pressure of O.sub.2 increases. This manifests itself as a decrease in the ignition temperature of the combustible material.
Existing methods which can be used to increase the partial pressure of O.sub.2 in the diesel exhaust gases include the introduction of additional air with the fuel. The quantity of air so introduced, however, is limited before a loss in engine efficiency results. Direct air introduction to the exhaust gases has been considered. By doing so, approximately 4 mols of N.sub.2 are introduced for each mol of O.sub.2. Any significant increase of O.sub.2 partial pressure is therefore accompanied by a major increase in the total volume of the exhaust gases and hence the back pressure on the engine. Such methods do not offer a practical solution for achieving lower ignition temperatures.
Examples of methods of increasing the oxygen are shown by U.S. Pat. No. 4,604,806. This patent generally discusses the problem of carbon particulate removal from the diesel engine exhaust gases and describes an air pressure regulating device which increases the pressure of combustion air supplied to the exhaust gases upstream of the filter used to remove the carbon particulates. Another U.S. Pat. No. 3,446,011 discloses the provision of air oxygen to accelerate combustion of the exhaust gases. In addition U.S. Pat. Nos. 4,622,810; 4,557,108; 3,808,805; and 3,486,326 disclose various systems for removing particulate exhaust gases of internal combustion engines in which air is supplied to the exhaust prior to treatment.
Certain fuel additives are known to promote low temperature ignition of the carbonaceous particulates produced during the operation of a diesel engine. However, separation of these components from the fuel can deposit within the engine and interfere with its operation as well as provide possible health hazards.
It is therefore an object of this invention to effect the ignition of the carbon at the lower temperatures prevailing in the exhaust gases and also to prevent excessive build up of carbon on the filter, thereby avoiding the risk of damage to the filter from high temperatures. The invention may be used in conjunction with other devices used for lowering the ignition temperatures of the carbon and promoting its combustion.